You should read a book. Dogs have helped the blind, the old, police, armies, and other people for a long time. All the while your cat pisses on the carpet, scratches the shit out of your door frame, and brings dead animals into the house.

It is all about insticts, I guess. The cat needs to have some natural instict to feel the mouse as a threat or food, but with this one something somewhere got wrong. But it is OK, the world needs more love and tenderness, not only scaring scenes of torturing the weaker ones or something more brutal, so I enjoyed this behaviour a lot!
See what am I talking about, only happiness and gentle care about cats at: http://www.o2cats.com